This invention relates to a jar for use in a downhole toolstring.
When exploring for oil or gas, or when preparing a wellbore for production, it is common practice in the oil and gas industries to employ strings of tools.
These toolstrings are lowered or driven into the wellbores, and include various devices that are activatable within the wellbores at the downhole location to carry out predetermined tasks.
The wellbore is rarely straight and parallel-sided.
This is sometimes because it is necessary to drill portions of the wellbore at angles to other parts thereof, in order to avoid difficult geological formations and more significantly to ensure that the wellbore perforates as much as possible of the hydrocarbon-bearing fields.
Furthermore, the pressures which exist below ground in wellbores can be very significant. These pressures can cause shales and other comparatively soft geological types to encroach into a wellbore, thereby rendering the wellbore non-uniform.
Another cause of non-uniformity of a wellbore is so-called “wash out”, caused when fluids in a surrounding rock formation cause decay and/or collapse of the wellbore.
All of the foregoing causes of non-uniformity can cause difficulty when attempting to operate exploration and/or production tooling within the wellbore.
For example, a common problem is for a toolstring that is being lowered into the wellbore on a wireline (i.e. a comparatively thick cable that supports, and sometimes conveys data transmission cables to, the downhole toolstring) to pass through a narrowed or deviated portion of the wellbore, and subsequently become stuck at that location as the wireline is being wound into a surface location to withdraw the toolstring.
Under such circumstances, there is a limit to the pull force that surface-located operators can apply via the wireline.
This is primarily because of the risk of breaking the wireline, thereby leaving the toolstring stuck in the wellbore.
Wireline used is usually either termed “slickline” comprising a single strand, common sizes being 0.108″ and 0.125″ diameter; or “braided line” comprising multi-strands of thinner wire which is wound or braided to give strength. This is available in common diameter sizes of 3/16″ and 7/32″ and sometimes larger. This type is stronger and more often utilized for “heavy duty” operations such as fishing.
Under such circumstances it is necessary to wind in the entire length of the wireline (over, perhaps, many of tens of thousands of feet), and then send into the wellbore a more robust cable carrying further tooling for cleaning the end of the broken wireline, and attaching to the toolstring for the purpose of attempting to withdraw it.
This practice suffers disadvantages, not least because it is time-consuming.
Since the operational time of an oil rig is typically costed in tens of thousands of dollars per day it is essential that rig operators recover stuck tooling as quickly as possible.
For this reason it has become commonplace to include a so-called “jar” in a toolstring.
In general terms, a mechanical spring jar is a device included in a toolstring that when needed utilities the limited pull force available via the wireline to cock a mass against a spring, and subsequently release it so that the energy resulting from tensioning of the wireline drives the mass into a part of the toolstring.
This imparts an impulse to the toolstring, which often is adequate to free the stuck tool.
Patents numbers U.S. Pat. No. 5,052,485 and U.S. Pat. No. 5,267,613 describe two known types of jar.
It is known to provide an adjustment mechanism in a toolstring jar, for adjusting the pull force needed to tension the wireline. Thus it is possible to match the force needed to operate the jar to the strength of the wireline being used and/or the mass of the toolstring, before the toolstring is inserted into the wellbore.
One known form of jar includes a cylindrical housing having a hollow, cylindrical interior.
Within the interior a jar mandrel and a latch sub are releasably secured together, with the jar mandrel located in use above the latch sub.
The latch sub includes a collar or other protuberance that bears against a compression spring defining a hollow cylinder. The latch sub and/or the jar mandrel extends through the center of the spring, the end of which opposite the collar bears against a further protuberance protruding from the wall of the hollow housing. The further protuberance and the collar between them define an elongated chamber for the compression spring.
The means of securing the jar mandrel and the latch sub together includes an annular array of latch keys that are moveable radially inwardly and outwardly relative to the jar mandrel.
A series of springs or other resiliently deformable members urges the latch keys to a radially outward position in which they engage a groove or recess formed in the radially inner surface at the upper end of a hollow, cylindrical latch sub. The groove is machined during manufacture of the latch sub, to define an annular shoulder of corresponding profile to the latch keys.
The interior of the housing includes one or more cam surfaces that, on movement of the jar mandrel upwardly in the housing, engage the latch keys.
This causes the latch keys to drive inwardly relative to the jar mandrel. This in turn causes their release from the latch fingers of the latch sub.
When the jar mandrel and latch sub are secured together any such upward movement of the jar mandrel involves similar movement of the latch sub. Therefore, the initial movement occurs against the force of the compression spring acting between the collar and the protuberance extending from the housing wall.
Thus when the cam surface causes release of the latch keys from the latch fingers, stored potential energy in the wireline reacts suddenly to drive the jar mandrel further upwardly within the housing, causing a hammer member secured to the jar mandrel to strike an anvil defined within the housing and thereby confer an upwardly acting impulse on the housing, and hence any further part of the toolstring secured thereto.
A portion of the jar mandrel at the upper end thereof protrudes via an aperture in the upper end of the housing.
This end of the jar mandrel includes a conventional rope socket for attachment to a wireline, such that when the toolstring is stuck in the bore the wireline is usable to draw the jar mandrel and latch sub upwardly against the action of the compression spring until the cam surface causes release of the latch keys from the latch fingers.
Typically the toolstring includes, located immediately below the rope socket, a number of weight bars. During the upward motion of the jar mandrel and latch sub the wireline stretches. When the latch keys release the resulting potential energy in the wireline converts to kinetic energy which accelerates the mass of the weight bars.
The rapid upward motion of the weight bars drives the hammer into FINNEGAN the anvil, to create the impulse on the stuck tool as aforesaid. It is, by this means, possible to confer significant impulses on the toolstring.
The length of the latch sub relative to the jar mandrel is effectively adjustable, by reason of its upper end passing through a bulkhead in the lower end of the housing, the dimensions of the latch sub above the bulkhead and the aperture through which it passes being such as to prevent withdrawal of the latch sub downwardly through the bulkhead.
The opposite (in use lower) end of the latch sub extends towards the open, lower end of the housing and is threaded. An adjuster nut is threaded onto the end of the protruding threaded portion. It is thereby possible to apply a spanner to the adjuster nut and drive it upwardly and downwardly relative to the housing, by turning the adjuster nut clockwise or anticlockwise.
Since the compression spring lies between the aforesaid bulkhead and a washer resting on the adjuster nut so as to encircle the latch sub, adjusting the adjuster nut in this way alters the length of the chamber containing the compression spring.
Such adjustment of the length of the chamber in turn alters the pre-load applied to the compression spring. This in turn affects the force needed to draw the latch sub and jar mandrel upwardly until the latch keys engage the cam surface. Thus it is possible to match the operating load of the jar to the strength of the wireline being used to lower and/or control the pull string; and/or to the mass of the toolstring, by altering the effective stiffness of the jar. When the level of pre-load is high, stretching of the wireline commences at lower wireline tension than when the pre-load is less (giving rise to a less stiff system overall).
However, the aforesaid method of adjusting the operating load of the jar is inconvenient.
This is principally because it is necessary to unscrew the jar from the toolstring in order to effect the adjustment.
This in turn involves withdrawing the toolstring, perhaps over the total depth of a long well, to a surface location. This may take several hours.
Thereafter it is necessary to remove the toolstring from the well; to disconnect the wireline from the upper end of the jar; and remove the jar from the toolstring. Only thereafter is it possible to apply the spanner to the adjuster nut in the free, lower end of the jar. Following these steps the time-consuming process of re-assembling the toolstring and lowering it back into the wellbore commences.
In view of the great cost of oil and gas rig downtime, there is a strong need for a more efficient method of adjusting the operating load of a mechanical spring jar.
Furthermore, in recent years the strength of the wirelines generally has increased.
This means that the wirelines are capable of operating the jars with ever larger springs and at ever increasing amounts of pre-load.
However, the latch sub shoulder and the latch keys limit the loads at which the jar mandrels and latch subs separate from one another.
In particular, a known latch key and latch sub groove combination FINNEGAN includes a latch key having a shaped outer surface that presents a recess having one or more upwardly facing shoulders. The latch sub groove includes a protuberance of complementary shape to the recess. The use of high loads in the wirelines causes the pairs of protuberances to slide one over the other thereby causing separation of the jar mandrel and latch sub even before the latch keys engage the cam surface.
Also, the repeated use of high loads causes premature wear in the latch keys and groove.
Thus, there is also a need in the design of a jar for a more effective arrangement for securing the components together before their intended separation.